This invention relates to the fabrication of structures formed particularly from aluminum or its alloys and is particularly directed to aluminum or aluminum alloy structures, such as metallic sandwich structures, fabricated by a method involving fine grain development of at least one component of the structure, in conjunction with roll bonding and superplastic forming.
Aluminum and its alloys have been formed into structural materials by diffusion bonding, and particularly roll bonding, and superplastic forming. However, roll bonding and superplastic forming are difficult to carry out for many aluminum alloys, particularly those which are heat treatable to high strengths. It is accordingly desirable that aluminum alloys have a fine grain structure in order to be more readily deformed and elongated during the roll bonding and superplastic forming procedure. However, although methods have been developed to impart a fine grain size to aluminum alloys, such procedures have not heretofore been successfully applied for selectively roll bonding and superplastic forming aluminum alloys into desired structures.
U.S. Pat. No. 4,092,181 to Paton, et al, describes a method of imparting a fine grain structure to precipitation hardenable aluminum alloys. The alloy is first heated to a solid solution temperature to dissolve the precipitating constituents in the alloy, and the alloy is then cooled, preferably by water quenching, to below the solution temperature. The alloy is then overaged to form precipitates by heating at above the precipitation hardening temperature for the alloy but below its solution treating temperature. Strain energy is introduced into the alloy by plastically deforming it at or below the overaging temperature used. The alloy is then held at a recrystallization temperature resulting in the development of the grains in a fine grain structure.
U.S. Pat. No. 4,222,797 to Hamilton, et al, discloses a method similar to that of No. 4,092,181 and describes additional conditions for minimizing the grain size. According to this patent, strain energy is introduced into the alloy by plastically deforming it in a temperature range of 380.degree. F. to 450.degree. F. to reduce its cross-sectional area a total of 40 percent minimum, at least 25 percent of the reduction in area being accomplished in a single continuous deformation operation.
U.S. Pat. No. 4,490,188 to Bampton discloses a method for imparting a fine grain structure to 2,000 and 7,000 series aluminum alloy. The alloy is solution treated and overaged to provide a suitable precipitate. It is then softened and stabilized so that it can be cold rolled at room temperature without cracking. After cold rolling the alloy is held at recrystallization temperature so that new grains are nucleated and grow to form a fine grain structure.
U.S. Pat. No. 4,434,930 to Trenkler, et al, describes a method for roll bonding and gas expanding metal sheets into composite structures. The process comprises providing at least two flat sheets of malleable metal, such as aluminum, and providing a selected pattern of stop-off material between at least two adjacent metal sheets, the stop-off material being thermally decomposable to generate a gas when raised above a given temperature, a first portion of stop-off material generating gas at the sintering temperataure and the second portion generating gas above a predetermined temperature higher than the sintering temperature. Such procedure, however, often results in separation and deformation of the sheets contiguous to the stop-off material.
Typically, a three-sheet truss core sandwich structure made using prior art methods has a thin center core sheet. The center sheet is thinner than the outer sheets so that it can be superplastically elongated to a greater degree than the outer sheets in order to achieve the desired central reinforcing structure without putting undesired grooves or pillows in the outer sheets. However, after superplastic forming, this central sheet was often too thin to provide the sandwich structure with the necessary compressive strength to prevent collapse of the structure upon the application of pressures of normal use. Thus, prior art methods have the disadvantage of typically being unable to produce sandwich structures having a strong central reinforcement structure unless excessively thick starting gauges are used for the outer sheets, often requiring expensive chemical thinning stages, after the superplastic forming stage, to achieve minimum weight of the part.
It is an object of the present invention to provide a method for producing aluminum structures from a plurality of heat treatable aluminum components or sheets, by imparting a fine grain structure to at least one of such sheets in conjunction with the steps of roll bonding and superplastic forming the sheets.
Another object of the invention is to provide an effective method for fabricating aluminum alloy structures, such as truss core sandwich structures, from a plurality of contiguous heat treatable aluminum alloy sheets, by imparting a desired fine grain structure to at least one of such sheets, particularly the center sheet, in conjunction with roll bonding and superplastically forming the aluminum alloy sheets.
Still another object of the invention is the provision of a method for roll bonding and superplastically forming a plurality of aluminum alloy sheets, including heat treating at least one of the sheets both prior to and following roll bonding to produce a fine grain structure which particularly facilitates the superplastic forming operation.